1. Field of the Invention
This invention relates broadly to field of semiconductor devices (and associated fabrication methodology) and, in particular, to semiconductor devices (and associated fabrication methodology) that utilize modulation doped quantum well heterojunctions to realize multiple-wavelength laser (detector) arrays.
2. State of the Art
Multi-wavelength laser arrays are attractive light sources for wavelength division multiplexing communication systems. Typically, such laser arrays are realized by vertical cavity surface-emitting lasers (VCSELs) fabricated into 2-D arrays. The emission wavelength of a VCSEL is usually determined during epitaxial growth by layer thicknesses.
In Wipiejewski et al., “Vertical-Cavity Surface-Emitting Laser Diodes with Post-Growth Wavelength Adjustment”, a vertical-cavity laser is provided that includes an active layer sandwiched between a bottom Distributed Bragg Reflector (DBR) mirror and a top DBR mirror. The active layer consists of three InGaAs quantum wells with GaAs barriers and AlGaAs cladding layers. On top of the top DBR mirror is formed a GaAs tuning layer, an SiO2 layer and reflective Au top layer. Etching of the GaAs tuning layer after epitaxial growth and before metal deposition sets the laser cavity length and corresponding emission wavelength of the vertical cavity laser. The emission wavelengths of individual lasers in a 2-D array VCSEL are controlled by adjusting the thickness of the GaAs tuning layer by a controllable etching process (utilizing anodic oxidation with in situ voltage monitoring and subsequent semiconductor oxide removal).
Although the vertical-cavity laser array of Wipiejewski et al. succeeds in providing in situ adjustment of the emission wavelength of the individual laser elements in a 2-D laser array, it has many disadvantages. For example, electrical contact is made through the GaAs tuning layer. Because the depth of the GaAs tuning layer varies from wavelength to wavelength, the threshold current of the devices of the array vary, which makes it very difficult to control the devices of the array. In addition, it is difficult to control the depth of the GaAs tuning layer when utilizing anodic etching as described, thus making it difficult to manufacture the array. Finally, in many applications (such as DWDM communication systems) there are significant cost advantages that arise by monolithic integration of a laser array with supporting electronic circuitry (e.g., laser drive circuitry), waveguides and/or other optoelectronic devices, and Wipiejewski et al. does not provide a mechanism for accomplishing such integration.
Wavelength division multiplexing communication systems also require multi-wavelength detection systems. Typically, such optical detection systems are realized by an optical demultiplexer (e.g., a fiber bragg grating or thin film optical filter) that separates the desired wavelength components in the incident light signal. The wavelength components are directed to a photodetector array. This approach is costly due to the high costs of packaging the optical demultiplexer with the photodetector array.
The state of the art in wavelength demultiplexing is performed by an element called the array wavelength grating (AWG). This is an element laid out in the plane of an integrated circuit that routes all wavelengths by waveguide into a free space region (parallel to the chip surface) from one side with a particular shape such that destructive interference takes place on the exit side and each wavelength is thereby guided to a unique output port. This arrangement is consumptive of real estate on the integrated circuit and is limited in wavelength resolution (i.e., the shape of the exit side will only allow a certain wavelength interval which is equivalent to the Q of an optically resonant filter).
Thus, there remains a need in the art for improved multi-wavelength laser/detection mechanisms in addition to multi-wavelength laser/detection mechanisms that are suitable for monolithic integration with a broad range of electronic circuitry (such as FETs and bipolar type transistors, logic, etc), waveguides and other optoelectronic devices.